This invention relates to a pilot valve for a control valve in a main flow line, and more particularly to a so-called no-flow pilot valve which is non-flowing during normal operation.
A non-flowing pilot valve does not have a continuous fluid flow during its normal operating or on position when the main control valve is in a static position and throttling. The elimination of a continuous fluid flow removes many of the problems in pilot valves having a continuous fluid flow such as, for example, the clogging of orifices and the like with entrained foreign matter in the flowing fluid, or icing resulting from wet gas applications. While a very small amount of fluid may be transmitted through the pilot valve during operation, this amount is insignificant.
More importantly in many respects, however, is the sensitivity or responsiveness of the pilot valve to changes in the sensed fluid pressures. The responsiveness of a non-flowing pilot valve to pressure changes in a sensed fluid is relatively constant over its operating range and a non-flowing pilot valve is not as affected by orifice size as is a flowing pilot valve. Thus, non-flowing pilot valves have a faster response to pressure changes in the sensed fluid as compared with pilot valves having a continuous flow of fluid therethrough.
Heretofore, such as illustrated in U.S. Pat. No. 3,572,359 dated Mar. 23, 1971, non-flowing pilot valves have been utilized with a control valve in a main flow line for regulating either downstream fluid pressure or upstream fluid pressure. The fluid pressure being sensed by the pilot valve is the fluid pressure being controlled in a no-flow pilot valve, and fluid flow through the pilot valve is permitted only when the dome pressure in the control valve is being changed. The sealing and seating arrangement of the valve members in U.S. Pat. No. 3,572,359 does not result in a pilot valve with a consistently high degree of sensitivity.
U.S. Pat. No. 2,736,337 issued Feb. 28, 1956, shows a pressure regulating device which may be used as a back pressure, pressure reducing, or liquid level control device to control fluid pressure. A pilot valve is shown which controls a main valve in a main pressure flow line. However, the regulating device is controlled by a pressure differential between the predetermined pressure and the pressure being controlled. The main control valve has a diaphragm which is acted upon by fluid pressure from the pilot valve, thereby to effect control of the fluid pressure in the flow line having the control valve therein.